In recent years, as a semiconductor material for short wavelength light-emitting devices, a GaN-based compound semiconductor material, which is a nitride-based semiconductor material, has been spotlighted. A GaN-based compound semiconductor is formed on a substrate made of sapphire single crystals, various oxides, a group III-V compound or the like using metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) or the like.
One of the characteristics of a gallium nitride-based compound semiconductor light-emitting device is low current diffusion in a horizontal direction. Due to this characteristic, current is introduced into only a semiconductor right below an electrode and light emitted from a light-emitting layer is interrupted by the electrode, thereby making it difficult to output the light from the light-emitting layer to the outside. For the purpose of avoiding this, such a light-emitting device is typically configured to include a transparent electrode as a positive electrode through which light is output.
The transparent positive electrode is made of Ni/Au, ITO (In2O3—SnO2) or the like, as is known in the art. Metal such as Ni/Au and the like has low contact resistance with a p-type semiconductor layer and low light transmittance. However, there is a problem in that although an oxide such as ITO has high light transmittance, it has high contact resistance.
There has been proposed a gallium nitride-based compound semiconductor light-emitting device including a positive electrode formed of a combination of a metal oxide layer having excellent conductivity, such as ITO or the like, and a contact metal layer (for example, see Patent Document 1, i.e. Japanese Patent Application Laid-Open No. Hei-9-129919).
However, in the gallium nitride-based compound semiconductor light-emitting device disclosed in Patent Document 1, although the contact metal layer used in the positive electrode lowers contact resistance with a p-type semiconductor layer, since the contact metal layer has low light transmittance, sufficient light output efficiency can not be obtained, which leads to low emission efficiency.